Fluidized bed catalytic cracking (commonly referred to as FCC) processes were developed during the 1940's to increase the quantity of naphtha boiling range hydrocarbons which could be obtained from crude oil. Fluidized catalytic cracking processes are now in widespread commercial use in petroleum refineries to produce lighter boiling point hydrocarbons from heavier feedstocks such as atmospheric reduced crudes or vacuum gas oils. Such processes are utilized to reduce the average molecular weight of various petroleum-derived feed streams and thereby produce lighter products, which have a higher monetary value than heavy fractions. Though the feed to an FCC process is usually a petroleum-derived material, liquids derived from tar sands, oil shale or coal liquefaction may be charged to an FCC process. Today, FCC processes are also used for the cracking of heavy oil and reduced crudes. Although these processes are often used as reduced crude conversion, use of the term FCC in this description applies to heavy oil cracking processes as well.
Differing designs of FCC units may be seen in the articles at page 102 of the May 15, 1972 edition and at page 65 of the October 8, 1973 edition of "The Oil & Gas Journal".
Other examples of FCC processes can be found in U.S. Pat. Nos. 4,364,905 (Fahrig et al.); 4,051,013 (Strother); 3,894,932 (Owen); and 4,419,221 (Castagnos, Jr. et al.) and the other FCC patent references discussed herein.
An FCC process of the most common design involves contacting a charge stock in a reaction zone with a finely divided solid catalytic material which is pneumatically conveyed through the reaction zone by a fluidizing medium. The fluidizing medium can include steam, light and vaporized feed components which are converted by contact with the catalyst. The catalyst is described as being in a fluidized state because it behaves as a fluid while it is transported by the fluidizing medium. Contact of the catalyst particles with the feed covers the catalyst with a hydrocarbonaceous material referred to as coke. Coke is a by-product of the cracking reaction and is comprised of carbon, hydrogen, and other materials present in the feed, such as sulfur. Coke blocks cracking sites on the catalyst and deactivates the catalyst. Such catalyst is generally referred to as spent catalyst. Therefore, after passage through the reaction zone, spent catalyst is transferred to a regeneration zone where the coke is removed from the catalyst by combustion. An oxygen-containing gas, typically air, is mixed with the catalyst in the regenerator at sufficient temperature to initiate the oxidation of the coke deposits to carbon monoxide and carbon dioxide. Removal of coke via oxidation reactivates the catalyst which is withdrawn from the regenerator and returned to the reactor to complete the continuous operation of the FCC unit.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods. However, the catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores and upon the external surface of the catalyst. Although the quantity of hydrocarbon retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modern FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. It is important to remove retained spent hydrocarbons from the spent catalyst for process and economic reasons. First, hydrocarbons that entered the regenerator increase its carbon-burning load and can result in excessive regenerator temperatures. Stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products. Avoiding the unnecessary burning of hydrocarbons is especially important during the processing of heavy (relatively high molecular weight) feedstocks, since processing these feedstocks increases the deposition of coke on the catalyst during the reaction (in comparison to the coking rate with light feedstocks) and raises the combustion load in the regeneration zone. Higher combustion loads lead to higher temperatures which at some point may damage the catalyst or exceed the metallurgical design limits of the regeneration apparatus.
The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, countercurrent to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and hydrocarbons which are absorbed on the catalyst.
Stripping of hydrocarbon vapors from the catalyst requires only contact of the catalyst with a stripping medium. This contact may be accomplished in a simple open vessel as demonstrated by U.S. Pat. No. 4,481,103.
In the past, the efficiency of catalyst stripping has been increased by using a series of baffles in a stripping apparatus to cascade the catalyst from side to side as it moves down the stripping apparatus. Moving the catalyst horizontally increases contact between it and the stripping medium. Increasing the contact between the stripping medium and catalyst removes more hydrocarbons from the catalyst.
As shown by U.S. Pat. No. 2,440,625, the use of angled guides for increasing contact between the stripping medium and catalyst has been known since 1944. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of similar stripping devices for FCC units are shown in U.S. Pat. Nos. 2,440,620; 2,612,438; 3,894,932; 4,414,100; and 4,364,905. These references show the typical stripper arrangement having a stripper vessel, a series of baffles in the form of frusto-conical sections that direct the catalyst inward onto a baffle in a series of centrally located conical or frusto conical baffles that divert the catalyst outward onto the outer baffles. The stripping medium enters from below the lower baffle in the series and continues rising upward from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 2,994,659 and the use of multiple linear baffle sections at different baffle levels as demonstrated by FIG. 3 of U.S. Pat. No. 4,500,423. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 where a quantity of fluidizing gas is admitted at a number of discrete locations.
In another form of gas-solid contact apparatus, presented in U.S. Pat. No. 2,460,151 (Sinclair), it has been shown that upward flowing reactants or vapors can be collected underneath a series of troughs and vented out the sides of the troughs through a series of louvers. However, this apparatus will not function in the manner of the previously described stripping devices since its arrangement provides a checkerboard pattern of open vertical passages through all trough levels. The '151 patent also attaches no particular importance to the design or provision of the louvers in the sides of the troughs.
In order to achieve good stripping of the catalyst and the increased product yield and enhanced regenerator operation associated therewith, relatively large amounts of stripping medium have been required. For the most common stripping medium, steam, the average requirement throughout the industry is well above 1.5 kg of steam per 1000 kg of catalyst for thorough catalyst stripping. The costs associated with this addition of fluidizing medium are significant. In the case of steam, the costs include capital expenses and utility expenses associated with supplying the steam and removing the resulting water via downstream separation facilities. Therefore, any reduction in the amount of steam required to achieve good catalyst stripping will yield substantial economic benefits to the FCC process.
It has now been discovered that good catalyst stripping can be achieved using conventional FCC stripping methods and devices with up to one-half or less of the stripping medium that was formerly used. These results are achieved by modifying the stripper grids in accordance with this invention.